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DYNAMICAL OCEANOGRAPHY
The tropical zone of the ocean basins has a special place within dynamical
oceanography. Through the existence of the trade winds, there is a large
transfer of momentum from the atmosphere to the ocean. In section 11.1
a few characteristics of the velocity, temperature and salinity fields are
presented together with the main issues which motivate this chapter. The
constant density and reduced gravity models on the equatorial β-plane are
formulated in section 11.2. With these models the physics of the Equatorial Counter Current is explained in section 11.3 and the zonal slope in
the equatorial thermocline in section 11.6. Equatorial waves are described
in section 11.4 and this material is needed in section 11.5 to describe the
wind-driven ocean circulation in an equatorial basin.
11.1. Characteristics
The annual mean wind stress over the Tropical Pacific is shown in Fig. 11.1.
Clearly, the trade winds are mainly zonal and directed from east to west. The
maximum amplitude of the zonal wind stress is about 0.1 Pa (note: 1 dyn/cm 2 =
0.1 Pa). At the equator, there is a small component of the meridional wind stress
with an amplitude of about 0.025 Pa. The structure of the winds is not symmetric
with respect to the equator since the convergence of the South Pacific trade winds
and North Pacific trade winds is located slightly north of the equator. This is
associated with the fact that the Intertropical Convergence Zone (ITCZ) is, on
average, located north of the equator.
As we saw in Fig. 5.13, there is a strong upwelling along the equator with a
maximum amplitude of about 2 m/day. The existence of this upwelling is directly
related to the Ekman transport associated with the trade winds. This transport is
90 ◦ to the right of the winds (and hence northward) in the northern hemisphere
and 90 ◦ to the left of the wind (and hence southward) in the southern hemisphere.
The resulting horizontal divergence of the mass transport has to be compensated
and hence upwelling results.
The equatorial surface flows have a rather complex structure as is illustrated
by the circulation by the snapshots of surface speed in the Pacific from the a high
resolution ocean (the NLOM) model (Fig. 11.2). Equatorial currents are highly
variable and intense in each of the three ocean basins with surface speeds up to
1ms −1 . In the Pacific, the westward South Equatorial Current has a maximum
north of the equator, which seems related to the asymmetry in the wind stress
(Fig. 11.1), while the North Equatorial Current has a maximum just north of 10 ◦ N.
A meridional section of the zonal geostrophic velocity at 155 ◦ W in the Pacific
(Fig. 11.3) shows the North and South Equatorial Current as strong westward
flows. The eastward Equatorial Counter Current is situated between these two
DYNAMICAL OCEANOGRAPHY
The tropical zone of the ocean basins has a special place within dynamical
oceanography. Through the existence of the trade winds, there is a large
transfer of momentum from the atmosphere to the ocean. In section 11.1
a few characteristics of the velocity, temperature and salinity fields are
presented together with the main issues which motivate this chapter. The
constant density and reduced gravity models on the equatorial β-plane are
formulated in section 11.2. With these models the physics of the Equatorial Counter Current is explained in section 11.3 and the zonal slope in
the equatorial thermocline in section 11.6. Equatorial waves are described
in section 11.4 and this material is needed in section 11.5 to describe the
wind-driven ocean circulation in an equatorial basin.
11.1. Characteristics
The annual mean wind stress over the Tropical Pacific is shown in Fig. 11.1.
Clearly, the trade winds are mainly zonal and directed from east to west. The
maximum amplitude of the zonal wind stress is about 0.1 Pa (note: 1 dyn/cm 2 =
0.1 Pa). At the equator, there is a small component of the meridional wind stress
with an amplitude of about 0.025 Pa. The structure of the winds is not symmetric
with respect to the equator since the convergence of the South Pacific trade winds
and North Pacific trade winds is located slightly north of the equator. This is
associated with the fact that the Intertropical Convergence Zone (ITCZ) is, on
average, located north of the equator.
As we saw in Fig. 5.13, there is a strong upwelling along the equator with a
maximum amplitude of about 2 m/day. The existence of this upwelling is directly
related to the Ekman transport associated with the trade winds. This transport is
90 ◦ to the right of the winds (and hence northward) in the northern hemisphere
and 90 ◦ to the left of the wind (and hence southward) in the southern hemisphere.
The resulting horizontal divergence of the mass transport has to be compensated
and hence upwelling results.
The equatorial surface flows have a rather complex structure as is illustrated
by the circulation by the snapshots of surface speed in the Pacific from the a high
resolution ocean (the NLOM) model (Fig. 11.2). Equatorial currents are highly
variable and intense in each of the three ocean basins with surface speeds up to
1ms −1 . In the Pacific, the westward South Equatorial Current has a maximum
north of the equator, which seems related to the asymmetry in the wind stress
(Fig. 11.1), while the North Equatorial Current has a maximum just north of 10 ◦ N.
A meridional section of the zonal geostrophic velocity at 155 ◦ W in the Pacific
(Fig. 11.3) shows the North and South Equatorial Current as strong westward
flows. The eastward Equatorial Counter Current is situated between these two